package com.juc.cas;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;

/**
 * @Author Zan
 * @Create 2024/5/11 10:21
 * @ClassName : SpinLockDemo
 * @Description :
 *          题目：实现一个自旋锁
 *          自旋锁好处：循环比较获取没有类似wait的阻塞
 *
 *          通过CAS操作完成自旋锁，A线程先进来调用myLock方法自己持有锁5秒钟，B随后进来后发现
 *          当前有线程持有锁，不是null，所以只能通过自旋等待，直到A释放锁后B随后抢到
 */
public class SpinLockDemo {

    AtomicReference<Thread> atomicReference = new AtomicReference<>();

    public void lock() {
        Thread thread = Thread.currentThread();
        System.out.println(Thread.currentThread().getName() + "\t --- come in");
        while (!atomicReference.compareAndSet(null, thread)) {
            try {
                TimeUnit.MILLISECONDS.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "\t --- wait...");
        }
    }

    public void unLock() {
        Thread thread = Thread.currentThread();
        atomicReference.compareAndSet(thread, null);
        System.out.println(Thread.currentThread().getName() + "\t --- task over, unLock...");
    }

    public static void main(String[] args) {
        SpinLockDemo spinLockDemo = new SpinLockDemo();

        new Thread(() -> {
            spinLockDemo.lock();
            try {
                TimeUnit.SECONDS.sleep(5);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            spinLockDemo.unLock();
        }, "A").start();

        // 暂停500ms，线程A先于B启动
        try {
            TimeUnit.MILLISECONDS.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        new Thread(() -> {
            spinLockDemo.lock();

            spinLockDemo.unLock();
        }, "B").start();
    }
}
